1. Field of the Invention
The present invention relates generally to thermal pellet incorporated thermal fuses and methods of producing thermal pellets used therefor, and particularly to thermal pellet incorporated thermal fuses employing thermoplastic resin as a thermosensitive material.
2. Description of the Background Art
A thermal fuse is generally classified into two types depending on the thermosensitive material used: a thermal pellet incorporated thermal fuse employing a non-conductive thermosensitive substance; and a thermal fuse using fusible alloy employing a conductive, low melting alloy. They are both a so-called, non-revertive thermal switch operating at a prescribed temperature to interrupt an electric current of equipment, apparatuses and the like or allow a conduction path to conduct to protect them as the surrounding temperature increases. It operates at a temperature determined by the thermosensitive material used. Typically, it is offered in products as a protective component functioning at a temperature ranging from 60° C. to 240° C. on a rated current ranging from 0.5 A to 15 A and it is an electrical protection method allowing an initial conducting or interrupt state for ordinary temperature to be inverted at a predetermined operating temperature to provide an interrupt or conducting state. The thermal pellet incorporated thermal fuse typically employs a non-conductive thermal pellet, which is accommodated in an enclosure having opposite ends with a lead respectively attached thereto, and includes a compression spring or the like that acts to exert pressure on a movable conductor. The thermal pellet is formed of a chemical agent having a prescribed melting temperature, which is molded into a granular state and then formed into a pellet.
Conventionally, practically used thermal pellet incorporated thermal fuses employ a thermal pellet formed typically of a single, organic chemical compound having a known melting point and made into a pellet, and blended with a binder to provide enhanced granulizability, a lubricant to provide uniform filling density, a pigment to classify types of thermal pellets, and the like. For example one such known thermal pellet incorporated thermal fuse using a single organic chemical compound is described in Japanese Patent Laying-Open No. 60-138819. This employs 4-methylumbelliferone as a pure chemical agent (used as an equivalent to an organic chemical compound) for its thermal pellet. Furthermore it has been known to mix two or more types of organic compounds to provide a melting point different from an initial source material. For example, Japanese Patent Laying-Open No. 2002-163966 and Japanese Patent Laying-Open No. 62-246217 both disclose that two or more types of known organic compounds can be mixed together to produce a eutectic mixture having a different melting point lower than that of an initial organic compound. The publications also describe that the obtained eutectic mixture maintains thermal stability and insulation property. In that case a thermal pellet incorporated thermal fuse employs a thermal pellet member formed of a pure chemical agent and it is said that if an unintended chemical agent is introduced the melting point varies. Accordingly thermal fuses typically employ a thermal pellet formed of a chemical agent such as certified chemically pure reagents or other similar reagents of high purity, and all of these are compounds of low molecular weight. Furthermore, these are all formed of a powdery chemical agent. If the agent is formed of a single type of agent it is molded into a pellet directly. If the agent includes two or more types of agents they are mixed together and then molded into a pellet. For insulation resistance at the time when a thermal pellet fuses, Japanese Utility Model Patent Publication No. 6-12594 proposes an approach to solve a problem associated with pelletization.
Conventionally as a thermosensitive substance a thermosensitive fusible substance including paraffin, a heat resistant non-conductive synthetic resin material, and the like has been used for a thermal fuse, as disclosed for example in Japanese Patent Laying-Open No. 50-138354 and Japanese Utility Model Laying-open No. 51-145538. They both utilize the fusibility that a thermosensitive material itself has. However, they are not commercially used as their selected materials' properties, structures and the like have issues to be addressed.
When a thermal pellet incorporated in a thermal fuse is exposed to high temperature close to its melting point, the thermal pellet can sublimate and thus be reduced in size. Furthermore by deliquescence the thermal pellet can be dissolved due to moisture, water and/or the like. Either case is a cause of a break of the thermal pellet incorporated thermal fuse. As such, the thermal pellet incorporated thermal fuse would hardly be thermally, physically or chemically sufficiently stable and is affected by environment. Furthermore, as it is formed of powder compacted and molded, it has insufficient strength and readily cracks, chips or the like while it is handled in a process for production. The thermal pellet incorporated thermal fuse also has a disadvantage in characteristic such as a low insulation resistance value after operation and for example Japanese Patent Laying-Open No. 2002-163966 and Japanese Utility Model Patent Publication No. 6-12594 raise such an issue. Furthermore in recent years there is an increased demand for a thermal fuse providing a quick response and hence increased response speed. To address the above described disadvantages individual approaches have been proposed. They are, however, individually unsatisfactory and there has not been a proposal in connection with a material that can satisfy all issues uniformly. For example, as will be described later in detail, material with a high insulation resistance value is not necessarily non-deliquescent. Rather, it suffers its higher dissolvability than other materials and it also disadvantageously readily sublimates.
The thermal fuses using the thermal pellet as described above employ a relatively pure chemical agent for the thermosensitive material, and this substance is granulated and molded into a predetermined form to provide the pellet. The pelletized material, however, readily softens, deforms, sublimates, deliquesces and/or the like as it is affected by environmental conditions, and there have been a large number of concerns associated with particular production process steps, and conditions for storage after production, and the like. For example if a pellet is molded from a material itself having deliquescent property, and it is exposed to external air, it deforms, dissolves and/or the like. Accordingly an extreme sealing management must be introduced to block external air. Furthermore, as the pellet is molded from powder, it is small in mechanical strength, and in assembling a thermal fuse a spring's force can deform the pellet, resulting in a defect. Furthermore, if a completed thermal fuse is stored at high temperature in high humidity, the pellet sublimates, deliquesces and/or the like, which can affect the product's longevity and also impair its electrical characteristics. Conventional thermal pellets employing chemical agents, low molecular weight chemical agents in particular, significantly soften and deform when they are exposed to high temperature and high humidity. The pellet thus diminishes in size, resulting in a contact dissociating disadvantageously. Accordingly there has been a need for a thermal pellet incorporated thermal fuse that is hardly affected in use by its surrounding environment, the variation thereof with the passage of time and the like, and also that has the pellet itself free of defects when it is stored in a severe atmosphere, exposed to high temperature and high humidity, toxic gas, and the like.
A conventional thermal fuse that uses resin material utilizes the resin material's fusibility. However, there is not any specifically described method to set an operating temperature, and the operating temperature's precision cannot be satisfactorily obtained. Furthermore, as an accurate operating temperature is not known, lack of practicality and other deficiencies exist, and there has been a demand for a thermal pellet incorporated thermal fuse overcoming such deficiencies. Furthermore, for improving the response speed there has also not been any specific solution indicated, and there is no thermal fuse providing a quick response in practical use. Furthermore, the resin that is used is difficult to select as it has a characteristic varying over a wide range. For example, if the resin material utilizes a melting point of crystalline thermoplastic resin, the melting point significantly varies with the resin's degree of crystallinity, composition and the like, and the fuse's operating temperature cannot be determined solely by the melting point. Without adjustment of an operating temperature, there is only limited thermoplastic resin that can be selected by depending solely on a melting point, and there has not been a material satisfactory for an operating temperature setting range required for practical thermal fuses. Furthermore, even crystalline thermoplastic resin having a melting point has a broad heat absorption peak, which is remote from a material having a narrow heat absorption peak that has been required for thermal fuses, and furthermore for amorphous thermoplastic resin a melting point itself cannot be utilized.